Jigsaw API nit: VersionQuery and Comparator<Version>

[Jonathan Gibbons]

Minor API comment:

It's weird having VersionQuery implement Comparator<Version>.  In 
principle, this means that different version query instances could 
provide different implementions of the comparator.  It would make more 
sense for Library or some other "more-singleton" item to provide the one 
true comparator for the system, without having to cons up VersionQuerys 
just to get the Comparator functionality.

-- Jon